1. Field of the Invention
This invention relates generally to electrical induction heating and more specifically to the electrical induction heating of workpieces of varying lengths.
2. Description of the Related Art
While induction heating has proven to be a valuable and efficient manner of heating workpieces, some problems are encountered when heating a static workpiece. In a static induction heating situation, the temperature distribution along the length of the workpiece is highly dependent upon the length of the induction coil and its relation to the workpiece length. In order to have a uniform distribution of the temperature along the length of the workpiece, the induction coils usually extend beyond both ends of the workpiece. This extension of the induction coil beyond the length of the workpiece is commonly referred to as "overlap".
The amount of overlap has a direct relationship to the temperature distribution in the workpiece. A large amount of overlap may cause the end of the workpiece to heat faster and hotter than the main body of the workpiece. An overlap that is too small will cause the end of the workpiece to be cooler than the main body.
The overlap is usually optimally adjusted by positioning the workpiece relative to the induction coil at one end and then adjusting the length of the coil to be energized on the other end by means of bolted taps along the coil length. The taps can then be connected to a source of voltage allowing only the desired portion of the induction coil to actually be energized.
When designing an induction coil for a specific application, the designer usually selects the number of inductor coil turns so that the required inductor voltage will match a standard power supply voltage. Large workpiece cross-sections require larger coil turn openings, which require a large voltage per turn. Induction coils operating at higher frequencies also require a high voltage per turn. Induction coils featuring large volts per turn usually have fewer turns, as too many turns can result in too high voltage for the coil. Induction coils with few turns means there are few points for electrically tapping the coil length. This presents a problem in that the length of the energized induction coil assembly can only be adjusted very coarsely by means of the taps. This results in an inaccurate control of the temperature distribution in the workpiece.
A reference directed toward controlling the length of an induction coil assembly for heating workpieces of varying length is U.S. Pat. No. 3,120,596.
Other problems associated with wide turns of induction coils is that of large radial flux losses and fabrication difficulties. Sometimes the radial flux losses may be avoided by using multiple narrower conductors operating in parallel. While the use of such multiple narrow conductors in parallel reduces the radial flux problem, it still provides too few electric taps and too coarse of a tapping adjustment. Should a person tap from only one turn from only one of the multiple conductors in parallel, this one conductor would then be shorter than the others and would have less impedance. The lower impedance would cause that conductor to draw more than its share of the total current. The higher share of current through this shortened conductor would result in the development of a "hot stripe" on the workpiece directly under the shortened turn.
The invention disclosed and claimed herein provides an electric induction coil assembly for heating workpieces of varying lengths which provides a more precise adjustment to the length of the coil assembly, thereby more accurately controlling the temperature distribution in the workpiece.